Power consumption is an important issue in computing platforms, especially in mobile platforms such as notebook computers and handheld devices which operate with a battery power supply. In most mobile computing platforms today, the processor is well power managed by periodic power cycling. The majority of other subsystems in the platform (e.g., devices, chipsets, memory, and clocks), however, are usually constrained to remain in a state of readiness even while the processor is powered down, due to the unpredictability of device interrupts and bus master traffic. For example, an interrupt may awaken a sleeping processor before its scheduled wakeup time, and a device needs to be ready in case the processor wants to access it. Peripheral devices also have to be ready to respond to bus cycles generated by any device acting as a bus master. These factors limit the opportunities for effectively reducing power consumption by only power cycling the processor.